The ever increasing demands for high capacity communications systems has resulted in a wide spread deployment of optical fiber networks across the world. A fundamental component used in such systems receives pulses of light and converts these into electrical signals. The pulses of light in such systems comprise a bit stream of information. This fundamental component employed in fiber optic networks is commonly known as an optical receiver module. Within the optical receiver, a photodetector is typically employed to receive light pulses and an amplifying circuit is employed for amplifying photocurrent generated within the photodetector.
Transimpedance amplifiers (TIAs) are typically used within optical receiver modules to amplify and transform weak photocurrents received from the photodetector, typically a photodiode or a PIN diode. The TIA amplifies and transforms the photocurrent into an output voltage that is further provided to other stages of the optical receiver module. Since TIAs are used to deal with both strong and weak photocurrents, noise in the resultant amplified and transformed voltage signal is typically a problem. Indeed, for those skilled in the art of the design of TIAs, it is well understood and appreciated that the noise introduced by the TIA, in many circumstances, limits the ability of the optical receiver module to faithfully reconstruct the intended stream of information. Furthermore, a relationship between the rate at which errors are produced by the receiver—often called the Bit Error Rate (BER), and the noise generated by the TIA can be shown. Thus, the optical receiver module needs to have low noise amplification performed on weak photocurrents in order to facilitate optical transmission of information. This is especially true in circumstances where the distance that the optical signal must travel is long and results in weak optical pulses at the receiver. It is known to those skilled in the art that long transmission distances—the distance between a transmitter and a receiver—serves to attenuate the initial transmitted optical signal strength and places a greater burden upon the receiver module to avoid errors. Furthermore, it is also known that cost of an optical communication system is reduced if a signal is transmitted along a longer length of optical fiber or, in the alternative, if less optical power is transmitted. Thus, providing low noise amplification for the TIA is important in order to reduce bit error rate (BER) of the received and amplified signal.
In optical receiver systems, the photodiode and TIA are typically co-packaged within a single module. After co-packaging, once the position of the photodiode is fixed in relation to a housing of the received module, an optical fiber is aligned to the photodiode in order to provide the pulses of light propagating in the optical fiber to the photodiode. Proper optical alignment of the optical fiber to the photodiode is critical in order to minimize optical coupling loss therebetween and in order to utilize a full dynamic range of the photodiode and TIA coupled therewith. In performing of optical alignment of the optical fiber to the photodiode, light is typically propagated through the optical fiber and a signal indicative of the quality of the alignment is provided from the TIA in order to obtain optimal positional alignment of the optical fiber.
In performing of this optical alignment, the light incident upon the photodiode always has a mean DC component and this DC component represents the mean signal strength of the optical signal as received by the photodetector and amplified by the TIA. Typically, the signal indicative of the quality of the alignment is in the form of a Received Signal Strength Indicator (RSSI) signal provided from an RSSI output port on the TIA. A magnitude of the RSSI signal represents the mean optical signal strength, which is used to align the photodiode detector to the optical fiber within the receiver module to achieve maximum responsivity during the manufacturing process or to provide an analog indication of the mean optical power incident upon the receiver for further processing during operation. This RSSI represents the mean DC current, or ratio of current, flowing through the photodiode detector into an input port of the TIA.
Often circuits that provide the RSSI signal are implemented externally from the TIA and are disposed in such a manner so as to monitor the DC current flowing at the cathode of a photodiode detector. This method unfortunately reduces the reverse bias voltage if a PIN diode detector is used. For single ended power supply operation this proves to be a problem, especially when 3.3V single ended power supply voltages are used.
An alternative approach to providing of the RSSI signal is to rectify the TIA output signal from the TIA and provide a Root Mean Squared (RMS) component of this signal that is representative of the RSSI. This, however, is not a true RSSI, since most TIAs exhibit automatic gain control (AGC) or signal limiting within the full optical dynamic range of the TIA and thus the RSSI is not a true representation of the DC current flowing through the photodetector and into the TIA.
A need therefore exists to provide a RSSI circuit that is representative of the coupling of the optical fiber to the photodiode without a reduction in the reverse bias voltage provided to the photodiode. It is therefore an object of the invention to provide an integrated RSSI circuit integrated with the TIA that does not reduce the reverse bias voltage when used with a photodetector in the form of a PIN photodiode. Furthermore, it is an object of the invention to provide an integrated RSSI circuit that can operate from the same voltage supply used by TIA. Furthermore, it is an object of the invention to provide an integrated RSSI circuit integrated with the TIA that does not introduce a sensitivity penalty due to a DC voltage offset between the active TIA and the reference TIA.